Counterfeiting is a growing problem. Recent estimates place the total amount of counterfeit goods sold each year at $150-200 billion worldwide, a 20-fold increase since 1970. The counterfeiting problem has become even more serious since the introduction of inexpensive, high quality color copiers, printers and scanners. These devices, some of which were unavailable several years ago, enable counterfeiters to reproduce the packaging and authentication features of many products. Currency has also been subject to counterfeiting using these devices.
Two broad categories of items that are frequently the subject of attempted counterfeiting are documents (including passports, title documents, identification cards, drivers licenses, and currency) and consumer goods. If a document is to be protected from counterfeiting by the use of a security laminate, the laminate typically must be transparent to enable the contents of the document to be seen. The security laminate should also be difficult to copy and tamper evident.
Minnesota Mining and Manufacturing Company of St. Paul, Minn. (3M) has sold a security laminate under the designation CONFIRM.TM. for twenty years. The product includes a monolayer of glass beads, which by virtue of its placement in the laminate retroreflects incident light. Although the CONFIRM.TM. product has many advantages over competitive security laminates, it requires the use of a verifier to illuminate the hidden image printed on the back side of the beads. It would be inconvenient, however, for a consumer to have to carry a verifier in order to authenticate money, or a consumer good, for example. In those and other circumstances, faster verification, without the need for a separate verifier, would be desirable.
Aluminized holograms came into considerable use because their authenticity may be verified without a separate verifier, but they have proven to be less secure because counterfeit holograms may be made relatively easily. In the commercial product authentication area, aluminum-backed holograms are becoming more popular, and may be applied to articles by sewing, gluing, molding, or the application of a tag or label. More recently, transparent holograms have gained considerable use as security laminates, because they provide transparency, they may be verified without the use of a separate instrument, and they are comparatively difficult to counterfeit.
An optical effect that has been found difficult to counterfeit is angularly sensitive reflective color filtration. This effect occurs when a layer of material acts as a color filter, reflecting incident light in one wavelength range and transmitting light in another wavelength range, with the wavelength ranges of reflection and transmission varying with changes in the incidence angle of the light. Typically, materials of this sort are made up of many thin layers, each of which is on the order of one quarter of the wavelength of light. These materials, which are referred to as "quarter wave stacks," are formed by the sequential deposition of isotropic materials with different refractive indices. For example, the Bank of Canada incorporates a silica-zirconia 5 layer stack into its currency. Sequential deposition is an expensive process, however, and thus the widespread use of quarter wave stacks for inexpensive consumer goods would also be impractical. To date, no known uses of quarter wave stacks for commercial product authentication have been found.
U.S. Pat. No. 5,149,578 (Wheatley et al.) mentions the use of color shifting films for security applications. However, Wheatley et al. points out that the visual effects of the disclosed films may not be sufficiently dramatic to be reliably perceived by some users, especially those having unusual or reduced color perception. More importantly, a film having an appearance that shifts from one color to another color may not be suitable for application to an item of value such as a document, where printing, graphics, or other indicia must be perceived through the film.
Other angularly sensitive materials, such as those made by Flex Products of Santa Rosa, Calif. for prototype currency for the U.S. Bureau of Engraving and Printing, were found to crack too readily, and were thereafter intentionally made into flakes that were incorporated into security inks. Forty countries have incorporated this type of ink into their currency, but so many colors are being fabricated that there is some confusion as to what colors are intended to represent authentic currency in each country, because there is no standard. Similar color shifting pigments are also being used in paints for commercial products, including automobiles. A disadvantage of these materials is that there does not appear to be any viewing angle at which they are transparent. As a result, when used as authentication layers, they can only be located on the item to be identified in an area which does not contain printed or other matter which is to be seen through the authentication layer.
Angularly sensitive diffraction films are disclosed in U.S. Pat. No. 4,484,797 (Knop et al.), wherein materials having particular combinations of refractive indices are embedded in a structure having other refractive indices, to produce a diffractive color filter which, in one example, changes color from reddish to white, and from reddish to green, when the angle of viewing changes from 0.degree. to 20.degree.. It would appear, however, that the complexity of the disclosed materials would require an expensive manufacturing process to produce a useful article. Moreover, the optical effect produced by these materials would appear to depend not only upon the angle of viewing, but also upon the orientation of the plane of viewing, which may make reliable authentication difficult for an untrained observer.
U.S. Pat. No. 5,089,318 (Shetty et al.) describes an iridescent film with thermoplastic elastomeric components, in which the film after being extruded is said to be brightly iridescent and was green and red when seen by reflection at perpendicular incidence. Films that appear colored at normal incidence, however, can obscure underlying images, which for security purposes can be undesirable.
A highly desirable feature of any authentication layer is that not only should the material itself not be counterfeitable, but that the anticounterfeiting feature of the material also provide within itself some means for incorporating readable or otherwise recognizable characters or other authenticating indicia in the material in a way which cannot easily be counterfeited. Conventional angularly sensitive materials are not believed to provide features of this type.
In light of the disadvantages of known products, it would be desirable to provide a low-cost article useful for the authentication of items of value, and specifically items having information provided thereon which should be viewed through the article.